1. Field of the Invention
The present invention relates to an ammoxidation catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane. More particularly, the present invention is concerned with an ammoxidation catalyst comprising a compound oxide and a silica carrier having supported thereon the compound oxide, wherein the compound oxide comprises molybdenum, vanadium, niobium and at least one element selected from the group consisting of tellurium and antimony, and wherein the alkali metal content of the ammoxidation catalyst is extremely small or substantially zero. By the use of the ammoxidation catalyst of the present invention, acrylonitrile or methacrylonitrile can be produced in high yield, as compared to the yield achieved by conventional ammoxidation catalysts containing a silica carrier. The present invention is also concerned with a process for producing acrylonitrile or methacrylonitrile by using such an excellent ammoxidation catalyst.
2. Prior art
A process for producing acrylonitrile or methacrylonitrile by ammoxidation of propylene or isobutylene has been well-known. Recently, as a substitute for such a process using propylene or isobutene, a process for producing acrylonitrile or methacrylonitrile by gaseous phase catalytic ammoxidation of propane or isobutane, i.e., by gaseous phase catalytic reaction of propane or isobutane with ammonia and molecular oxygen has attracted attention.
With respect to catalysts for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation, a number of proposals have been made. For example, as a catalyst for use in the ammoxidation of an alkane, oxide catalysts containing molybdenum, vanadium, niobium and tellurium are disclosed in EP 0 529 853 B1, U.S. Pat. No. 5,049,692, U.S. Pat. No. 5,231,214, U.S. Pat. No. 5,422,328, Unexamined Japanese Patent Application Laid-Open Specification Nos. 7-144132, 7-289907, 8-57319, 8-141401 and 10-28862.
Further, oxide catalysts containing molybdenum, vanadium, niobium and antimony are disclosed in, for example, EP 0 767 164 Al, U.S. Pat. No. 4,760,159, U.S. Pat. No. 4,797,381, Unexamined Japanese Patent Application Laid-Open Specification Nos. 5-213848 and 10-28862.
Thus, in the art, an oxide catalyst containing molybdenum, vanadium, niobium and at least one element selected from the group consisting of tellurium and antimony has attracted attention as a promising ammoxidation catalyst for producing an unsaturated nitrile from an alkane in high yield.
Generally, when an unsaturated nitrile, such as acrylonitrile or methacrylonitrile, is produced by ammoxidation on a commercial scale, the ammoxidation is conducted using a fluidized-bed reactor. With respect to a catalyst for the ammoxidation using a fluidized-bed reactor, it is necessary for the ammoxidation catalyst to have a high attrition resistance. For this reason, an ammoxidation catalyst conventionally has a structure wherein a compound oxide is supported on a silica carrier so as to provide an attrition resistance. However, with respect to conventional ammoxidation catalysts which comprise a compound oxide and a silica carrier having supported thereon the compound oxide, wherein the compound oxide contains molybdenum (Mo), vanadium (V), niobium (Nb) and at least one element selected from the group consisting of tellurium (Te) and antimony (Sb), there has been a problem in that, when the silica carrier is employed in an amount necessary for imparting a satisfactory attrition resistance to the catalyst, the yield of the desired unsaturated nitrile is likely to become low (the reason for this problem has not yet been elucidated). Therefore, various attempts have been made for solving this problem (i.e., for achieving a high yield of the desired unsaturated nitrile in the ammoxidation of an alkane even when using an oxide catalyst comprising a silica carrier having supported thereon a compound oxide of the above specific metallic elements).
For example, Unexamined Japanese Patent Application Laid-Open Specification No. 8-57319 discloses a method for producing an ammoxidation catalyst for use in producing acrylonitrile from propane, wherein the catalyst comprises a silica carrier having supported thereon a compound oxide of Mo, V, Nb and Te, in which the silica carrier is present in an amount of from 30 to 50% by weight, based on the total weight of the compound oxide and the silica carrier. In this prior art technique, for improving the acrylonitrile yield in the ammoxidation of propane, the catalyst is treated with an acid in order to activate the catalyst. However, this technique is disadvantageous not only in that a cumbersome operation for the acid treatment of the catalyst is required, but also the waste acid produced by the acid treatment of the catalyst must be disposed of properly. Thus, this prior art technique is disadvantageous from a commercial viewpoint.
Therefore, it has been desired to develop an improved ammoxidation catalyst comprising a silica carrier having supported thereon a compound oxide of Mo, V, Nb and at least one element selected from the group consisting of Te and Sb, which is not only effective for producing acrylonitrile or methacrylonitrile in high yield, as compared to the yield achieved by conventional ammoxidation catalysts containing a silica carrier, but also can be easily produced, so that it is advantageous from a commercial viewpoint.